The Selection of Abrasive Materials and Tools of Valves

Oct 13, 2021

Alumina (AL2O3)
Alumina (AL2O3) has good hardness and is widely used, which is usually used to grind workpieces made from cast iron, copper, steel and stainless steel.

Silicon Carbide (SiC)
Silicon carbide has two colors, that is, green and black, and its hardness is higher than that of alumina. Green silicon carbide is suitable for grinding hard alloys, while black silicon carbide is used for grinding brittle and soft materials, such as cast iron and brass.

Boron carbide (B4C)
The hardness of Boron carbide (B4C) is worse than diamond powder but better than silicon carbide. It is mainly used to replace diamond powder to grind hard alloys and hard chrome-plated surfaces.

Chromium oxide (Cr2O3)
Chromium oxide is a kind of extremely fine abrasive material with good hardness. Chromium oxide is often used in the fine grinding of hardened steel, and it is generally used for polishing.

Iron oxide (Fe2O3)
Iron oxide is also a very fine abrasive material for valves, but its hardness and grinding effect are worse than chromium oxide; its use is the same as chromium oxide.

Diamond powder
Diamond powder is the hardest abrasive material with good cutting performance and is especially suitable for grinding hard alloys.

The classification and application ranges of commonly used abrasive materials are shown in the following table.

Series	Abrasive materials	Features	Ranges of applications
Aluminium oxide series	Brown corundum	It is sepia and has good hardness, good toughness, and low prices.	It is used for rough grinding cast iron, green steel, copper and stainless steel. It can also be used for fine grinding when the requirement is not high.
	White corundum	It is white. It has better hardness than brown corundum, but it has poor toughness.
	Single crystal corundum	It is light yellow or white with spherical particles, which has better hardness and toughness than white corundum.
	Chrome corundum	It is rose-bengal or prunosus, and has better toughness than white corundum.
Carbide series	Black silicon carbide	It is black and glossy. It has better hardness than white corundum. It has poor toughness.	It is suitable for grinding cast iron, brass, and bronze.
	Green silicon carbide	It is green, which has hardness only next to boron carbide and carbite.	It is suitable for grinding hard alloys.
	Boron carbide	It is black. Its hardness is only next to carbite. It has good wear resistance, high prices. It can replace carbite partially.	It is suitable for grinding hard alloys and hard chromium surfaces.
Carbite series	Man-made carbite	It is light green, black or white. It has good hardness and high prices.	It is good for grinding materials with good hardness such as hard alloys and glass.
	Natural carbite	It is gray or light yellow. It has the best hardness, and it is expensive.
	Chromium oxide	It is dark green and has good hardness.	It is suitable for fine grinding and polishing steel and stainless steel.
	Iron oxide	It is crimson, and softer than chromium oxide.

In addition, the thickness of the abrasive particle size has a significant effect on the grinding efficiency and surface roughness after grinding. In the rough grinding process, there are low requirements for the surface roughness of the valve. In order to improve the grinding efficiency, coarse abrasives should be used. In the fine grinding process, the grinding allowance is small and the surface roughness of the workpiece is required to be high. Fine-grained abrasives can be used.

When the sealing surface is roughly ground, the abrasive grain size is generally 120# to 240#. As for fine grinding, it is W40 to 14.

The abradant is made by adding kerosene and engine oil directly to the abrasive material. The abradant made by blending 1/3 kerosene with 2/3 engine oil and abrasive is suitable for rough grinding; the abradant made by blending 2/3 kerosene and 1/3 engine oil and abrasive can be used for fine grinding.

When grinding workpieces with good hardness, using the above-mentioned abrasives is not ideal. At this time, three abrasives mixed with heated lard can be adopted, and they will form paste after cooling. When using, add some kerosene or gasoline and mix them thoroughly.

The selection of grinding tools
Valves cannot be directly ground due to the different degrees of damage to the sealing surface of the valve disc and valve seat. Instead, a certain number and specifications of artificial valve discs (grinding heads) and artificial valve seats (grinding seats) specially made in advance are used to grind the valve seat and disc.

The grinding head and seat are made from ordinary carbon steel or cast iron, and the size and angle should be equal to the valve disc and valve seat placed on the valve.

If the grinding is done manually, various grinding rods are required. Grinding rods and grinding tools must be assembled properly and not skewed. In order to reduce labor intensity and speed up the grinding speed, electric grinders or vibration grinders are often used for grinding.